1. Field of the Invention
The present invention is directed to the removal of hydrogen sulfide from a gaseous stream, and particularly the selective removal of hydrogen sulfide. It has long been known that hydrogen sulfide, which can be present in waste gases liberated during chemical and industrial processes, such as the pulping of wood, natural gas and crude oil production and petroleum refining, has an offensive odor, may damage vegetation and painted surfaces, and can constitute a significant health hazard to wildlife and humans. Government enforced regulations have imposed ever lower tolerances on the amount of hydrogen sulfide which can be vented to the atmosphere, in some localities requiring removal of virtually all of the hydrogen sulfide.
In general, the absolute quantities of hydrogen sulfide are not very high. For example, stack gases resulting from the concentration of waste pulping liquor from the craft pulping process contain about 500-2000 ppm of hydrogen sulfide. See U.S. Pat. No. 3,071,433 to Dunn. However, hydrogen sulfide odor can be detected by humans at concentrations as low as 0.01 ppm. Thus, a process for removal of hydrogen sulfide must be extremely efficient.
Carbon dioxide often is found along with hydrogen sulfide in the gas streams. However, it is desirable to remove only the hydrogen sulfide and not the carbon dioxide, to prevent unnecessary consumption of the solution which removes the hydrogen sulfide.
2. Description of the Prior Art
In the past, it has been known that hydrogen sulfide can be removed selectively from a gas stream containing carbon dioxide by using an alkaline liquid, such as caustic soda, and maintaining a very short contact time, about 0.02 sec. See U.S. Pat. No. 2,747,962 to Heitz, et al. It is also known that an alkaline solution can be sprayed countercurrent to a flow of waste gas to remove hydrogen sulfide. See U.S. Pat. No. 2,083,213 to Baehr, et al and Hohlfeld, "Selective Absorption of H.sub.2 S From Sour Gas", Society of Petroleum Engineers, Paper 7972, Copyright 1979 to the American Institute of Mining, Metallurgical and Petroleum Engineers, and presented at the California Regional Meeting of the Society of Petroleum Engineers held April 18-20, 1979 Ventura, Calif. A shorter version of the Hohlfeld paper appears at pages 129-130 of the Oct. 15, 1979 issue of the Oil and Gas Journal.